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PL2SP

       SUBROUTINE  PL2SP (PH,TH,VISBLE)
 C$    (2-D Spherical Polar Pen Movement)
 C$    Change the  angular  variables  (PH,TH)  to  the  Cartesian
 C$    coordinates X,Z so as to define directly in spherical polar
 C$    coordinates points which lie upon the surface of a constant
 C$    sphere and graph their projection  on the X-Z plane,  where
 C$    the Z axis is vertical, and  the X axis is positive to  the
 C$    right.  Points with positive Y values lie behind the  plane
 C$    and are marked as invisible.  (PH,TH) are all assumed to be
 C$    scaled to  the unit  interval.  The  Cartesian  coordinates
 C$    (X,Y,Z) are adjusted  to the  unit interval  and passed  to
 C$    LINA2/MOVA2 as (X,Z), so that  the default view plane  (the
 C$    X-Y plane) will  receive the  image.  Spherical  coordinate
 C$    ranges are
 C$
 C$    0 .LE. RHO .LE. infinity
 C$    0 .LE. PHI .LE. 2*pi
 C$    0 .LE. THETA .LE. pi
 C$
 C$    See H. Margenau  and G.M. Murphy,  "Mathematics of  Physics
 C$    and Chemistry", 2nd  Ed., Van  Nostrand (1956),  Vol 1,  p.
 C$    177.  These are related to the Cartesian coordinates by
 C$
 C$    X = RHO*SIN(THETA)*COS(PHI)
 C$    Y = RHO*SIN(THETA)*SIN(PHI)
 C$    Z = RHO*COS(THETA)
 C$
 C$    The coordinate surfaces are
 C$    (1) Concentric spheres about the origin (RHO = constant)
 C$    (2) Right circular cones with apex at the origin and axis
 C$        along Z (THETA = constant)
 C$    (3) Half planes from the Z axis (PHI = constant).
 C$
 C$    To  obtain  coordinates  (PH,TH)  expressed  on  the   unit
 C$    interval, (PHI,THETA) are transformed as follows:
 C$
 C$    PH = PHI/TWOPI
 C$    TH = THETA/PI
 C$    (09-APR-82)